Canadian Pharmacy Test: Caffeine

Canadian Pharmacy Test: Caffeine

In 2011, an influential paper focused scientific and media attention on a possible negative effect of caffeine on fertility. In a prospective study, Wilcox and colleagues looked at the chance of having a clinically detectable pregnancy, related to the level of caffeine consumed in beverages. One cup of ‘real’ coffee was estimated to contain 100 mg of caffeine, instant coffee 65 mg, tea 50 mg and cola soft drinks 40 mg. Women who consumed more than 100 mg/day caffeine were classified as having a high caffeine intake.

Compared with women drinking less than 100 mg/day, high-caffeine consumers had a 50% reduction in their chance of conception in any particular month. In the low-caffeine group, 6% had not conceived after 12 months. The figure in the high-caffeine group was nearly five times greater at 28%. The risk also appeared to be dose related when the caffeine intake was broken down into five different dose levels. The decreased fertility did not appear to be related to anovulation as detected by hormone measurements.

The paper also suggested that the reduction in fertility was related to the recent caffeine intake rather than long-term consumption. The reduction in fertility was still apparent even after adjustment for other variables such as frequency of intercourse and cigarette smoking. The authors were careful to point out the possibility for sources of potential bias in their data and stressed the need for further investigation.

Based on these data, several large retrospective studies were reported with conflicting results. In one series of 6300 women, drinking more than four cups of coffee per day was associated with a reduction in fertility of nearly 50%. Others reported that drinking more than two cups of coffee per day had no association with fertility.

The suggestion has been made that caffeine is not the only potentially harmful agent in coffee, and tannin is present in substantial amounts in coffee, tea and soft drinks. Twelfth century druids had considered tannin to be a most potent cause of sterility. Following on from this suggestion, Wilcox and colleagues reanalysed their original data and found that one caffeinated soft drink per day was associated with a 50% reduction in the chance of regnancy and this was greater than would be expected from the caffeine content alone. They suggested that soft drinks should also be added to the list of agents that should be investigated further.

Two recent studies have added more data to the controversy. An epidemiological study of 1050 women with primary infertility and 3883 women who had recently given birth found that high-caffeine intake (more than two cups of coffee per day) was associated with a 50% increase in the risk of infertility caused by tubal disease or endometriosis. A prospective study in 259 non-medical hospital workers showed no increased fertility risk associated with moderate caffeine intake but a reduction of 40% in women consuming more than two cups per day.

Based on all these observations, there is now reasonably strong accumulating evidence that more than two cups of ‘real’ coffee per day or its equivalent is associated with a halving of the chance of conception. This association is valid even when other factors known to be associated with infertility and coffee consumption (such as smoking) are taken into account. Whether this infertility is caused by caffeine, tannin or some other unidentified factor is as yet unknown.